Step 1: Caramelize the Onions
- In a skillet, heat butter and olive oil over medium-low heat.
- Add onions, salt, and sugar. Cook slowly for 20–25 minutes, stirring occasionally, until golden brown and jammy.
- Stir in thyme and balsamic vinegar. Set aside.
Step 2: Build the Sandwich
- Butter one side of each bread slice.
- On the unbuttered side, layer caramelized onions, shredded Gruyère, and mozzarella.
- Top with another slice of bread, buttered side out.
Step 3: Grill to Perfection
- Heat a skillet or griddle over medium heat.
- Place sandwich in pan and cook until golden brown and crisp, about 3–4 minutes per side.
- Press gently with a spatula to help cheese melt evenly.
Step 4: Serve
Slice in half and serve hot — preferably with a bowl of tomato soup or a small side salad.
Serving Suggestions
- Pair with French onion soup for the ultimate cozy combo.
- Serve with red wine (like Pinot Noir or Merlot).
- Add a side of crispy fries or salad for a bistro-style meal.
Tips & Variations
- Cheese swap: Try white cheddar, fontina, or Havarti.
- Extra indulgent: Add a spoonful of beef broth concentrate or onion soup mix to the caramelized onions.
- Make it meaty: Add thinly sliced roast beef or ham.
- Crunch upgrade: Sprinkle a little Parmesan on the outside of the bread before grilling.
Why This Recipe Works
The deeply caramelized onions mimic the soul of French onion soup, while the melty cheese brings gooey comfort. Toasted, buttery bread adds crunch, making this a sandwich that’s both gourmet and nostalgic.
Final Bite
Golden, crispy, cheesy, and loaded with caramelized onion goodness, this French Onion Grilled Cheese is comfort food at its best. One bite, and you’ll never look at regular grilled cheese the same way again.
So grab those onions, melt that butter, and let’s build the grilled cheese of your dreams.